In the preparation of elastomeric polyurethanes, a high molecular weight organic compound containing active hydrogen atoms, such as a polyester, a polyether or the like, is reacted with an organic polyisocyanate and if desired, a chain-extending agent such as, for example, an amine, a glycol, water or the like, to produce an elastomeric product. The reaction mixtures can be fabricated into the final desired shape by various techniques. In one case, the reaction compositions can be mixed together simultaneously or in steps and then immediately cast into a mold having the desired configuration. Alternatively, the reaction compositions can be mixed either simultaneously or in steps, permitted to react for a short time and then by interrupting the reaction, a pseudo thermoplastic results which can be fabricated by all the techniques suitable in the thermoplastic art. Still in another method, an interpolymer, generally referred to as a millable gum, can be prepared and this gum worked on a mill such as that used in the rubber industry, whereupon, additional ingredients and reactants, such as pigments, fillers and further quantities of curing agents such as polyisocyanates, sulfur, peroxides or the like can be added in order to effect a cure.
In all of these processes, where the reaction mixture is subjected to a subsequent shaping technique, and particularly where thermoplastic techniques are used, polyurethanes in general, do not always exhibit good processing characteristics. For example, if an extrusion or injection molding is to be made, it is sometimes difficult to fabricate the desired article. Necessarily, the art has looked to various techniques of improving the processing of polyurethane elastomers. Characteristic of these techniques are the addition of processing aids such as poly-lower alkyl styrene resins (U.S. Pat. No. 3,385,909), urea-type compounds (U.S. Pat. No. 3,321,433), and polyolefins (U.S. Pat. Nos. 3,351,676 and 3,310,604).
Additionally, blends of thermoplastic polyurethanes with other thermoplastic materials are known. The other thermoplastic material is usually chosen so as to enhance one or more of the properties of the polyurethane elastomer. Thus, e.g., graft copolymers of polybutadiene, styrene and acyrylonitrile have been added to thermoplastic polyurethanes in order to increase the tear strength thereof (see, e.g., U.S. Pat. No. 3,049,505). Additionally, various thermoplastic polyoxymethylenes have been added to thermoplastic polyurethane in order to reduce the permanent elongation thereof, to increase the notch toughness thereof, and to improve the resistance thereof to degradation under different environmental conditions (see, e.g. Canadian Patent No. 842,325 and British Patent No. 1,017,244). Finally, thermoplastic polycarbonates have been blended with thermoplastic polyurethanes to increase the hardness, and set and tear strength of the thermoplastic polyurethane and to provide materials having improved resistance to environmental stress crazing and cracking (see, e.g. U.S. Pat. No. 3,431,224 and U.S. application Ser. No. 705,745, filed July 15, 1976).
Although the blends described above represent an important advance in the art, all suffer from various problems during the processing thereof. All of the blends noted have one characteristic in common. Specifically, all phases in the blend are thermoplastic and melt at some point during the molding and/or extrusion thereof. Necessarily this means that the ultimate properties of the final product will depend upon the degree of mixing of the components. As with the pure thermoplastic polyurethane, such blends also suffer from various processing problems such as non-uniformly melt flow and the like.